EFFECT OF MICRON AND NANO MgAl2O4 SPINEL
ADDITION ON THE PROPERTIES OF MAGNESIA-CARBON
REFRACTORIES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Master of Technology (Research)
in
Ceramic Engineering
By
RASHMI REKHA DAS
Department of Ceramic Engineering
National Institute of Technology
Rourkela
October 2010
EFFECT OF MICRON AND NANO MgAl2O4 SPINEL
ADDITION ON THE PROPERTIES OF MAGNESIA-CARBON
REFRACTORIES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Master of Technology (Research)
in
Ceramic Engineering
By
RASHMI REKHA DAS
Under the Guidance of
Dr. Bibhuti Bhusan Nayak
and
Dr. Sukumar Adak
Department of Ceramic Engineering
National Institute of Technology
Rourkela
October 2010
CONTENTS Page No
Abstract i Acknowledgements ii List of Figures iii List of Tables iv Chapter 1 GENERAL INTRODUCTION 1-8 1.1 Introduction 2 1.2 MgO-C refractory and its application in ladle 3 1.3 Role of spinel in MgO-C refractory 5 1.4 Role of ceramic nanoparticles in refractory industry 6 1.5 Organization of the thesis 8
Chapter 2 LITERATURE REVIEW 9-22 2.1 Technological evolution of MgO-C refractories 10 2.2 Selection of raw materials 11 2.3 Role of micron-sized, stoichiometric and in-situ spinel in MgO-C
brick 15
2.4 Mechanisms of corrosion in MgO-C bricks 17 2.5 Effect of nanoparticles on the properties of MgO-C refractories 19 2.6 Synthesis of MgAl2O4 spinel using different chemical routes 20 2.7 Summary of literature 21 2.8 Objectives of the present studies 22
Chapter 3 EXPERIMENTAL WORK 23-32 3.1 Raw materials and fabrication of micron and nano spinel added
MgO-C brick 24
3.2 Synthesis of MgAl2O4 spinel nanopowders 27 3.3 General Characterization 28 3.3.1 AP, BD and CCS 28 3.3.2HMOR 28 3.3.3 MOE 29 3.3.4 TSI 29 3.3.5 Oxidation resistance 29 3.3.6 Rotary slag corrosion test for micron sized spinel added
MgO-C bricks 30
3.3.7 Static crucible slag corrosion test for nano sized spinel added MgO-C bricks
31
3.3.8 Pore size distribution 31 3.3.9 Thermal 31 3.3.10 Surface area 31 3.3.11 Phase analysis 32 3.3.12 Microstructure 32
Chapter 4 RESULTS AND DISCUSSION 33-54
4.1 Physical and chemical properties of micron-sized MgAl2O4 spinel added MgO-C bricks
34
4.1.1 AP, BD and CCS (before and after coking) 34 4.1.2 HMOR and TSI 35 4.1.3 Oxidation resistance 37 4.1.4. Rotary slag corrosion 38 4.1.5 Corrosion 39 4.1.6 Pore size distribution 40 4.1.7 Microstructure 41 4.1.8 Summary 44
4.2 Characterization of MgAl2O4 spinel nanopowders synthesized by citrate-nitrate method
45
4.2.1. Thermal analysis 45 4.2.2. Structure and microstructure 46 4.2.3 Surface area 47 4.2.4 Summary 47
4.3 Physical and chemical properties of without, standardized and nano-sized MgAl2O4 spinel added MgO-C refractory
48
4.3.1 AP, BD and CCS (before and after coking) 48 4.3.2 HMOR and TSI 49 4.3.3 Oxidation resistance 50 4.3.4. Static crucible slag corrosion 50 4.3.5 Corrosion 51 4.3.6 Pore size distribution 53 4.3.7 Microstructure 53 4.3.8 Summary 54
Chapter 5 CONCLUSIONS 55-57
SCOPE FOR FUTURE WORK 57 References 58-69 Curriculum Vitae
ABSTRACT Magnesia- carbon (MgO-C) refractory bricks have been used in the slag line of ladles
due to its superior slag penetration resistance and excellent thermal shock resistance at high temperatures. However, the life of this bricks has become limited on prolonged use due to its poor oxidation resistance as well as low strength at high temperatures. Thus, the physical and chemical properties of MgO-C refractories could be improved by the addition of suitable additives in micron or nano range. Magnesium aluminate (MgAl2O4) spinel has been recognized as one of the most effective refractory material due to its excellent wear and slag resistance. The particle size distribution of MgAl2O4 spinel is also important factor that influence both the physical and chemical properties of refractories. Hence, the present work deals with the improvement of the physical and chemical properties of MgO-C refractories with the addition of MgAl2O4 spinel in micron and nano range.
In this work, a set of experiments was carried out in order to standardize the type and amount of preformed spinel addition in MgO-C refractory system. Here, micron-sized MgAl2O4 spinel in three different commercially available grades such as near stoichiometric (AR-78), alumina rich (AR-90) and magnesia rich (MR-66) have been used during fabrication of MgO-C bricks. Micron-sized spinel added MgO-C bricks with sixteen compositions have been fabricated using different raw materials such as fused magnesia (FM97LC), flake graphite, resin, pitch and Al-metal powder. The micron spinel content was varied from 0 to 25 wt % with the incremental addition of 5 wt % in MgO-C bricks. It was observed that 10% AR-78 spinel added MgO-C bricks exhibits better corrosion and oxidation resistance as compared to that of AR-90 or MR-66 spinel added MgO-C bricks. HMOR and TSI were higher for AR-78 (10 wt %) spinel added MgO-C bricks. From the microstructure, it was observed that the dissolution of MgO grains into slag was less and carbon retention was more for AR-78 spinel added bricks as compared to without spinel added bricks. The standardized type and amount of spinel (10 wt % AR-78) was then taken in order to compare and carry out the second set of experiments. In this experiment, the effect of without, standardized micron-sized (10 wt % AR78) and nano-sized MgAl2O4 spinel added MgO-C bricks properties are correlated.
Nano-sized MgAl2O4 spinel has been prepared using citrate-nitrate method and calcined at 800 °C to get a cubic phase. These calcined spinel powders have been added with different weight percentage such as 0.1, 0.5, 1 and 1.5 in MgO-C bricks.
The average pore diameter of nano spinel added brick was lower as compared to AR-78 spinel added MgO-C bricks. Nano spinel addition restricts the dissolution of MgO grains and retains the carbon in the matrix. It was observed that with addition of 0.5 to 1 wt % nano MgAl2O4 spinel gives better HMOR and TSI as well as oxidation and slag corrosion resistance as compared to 10 wt % AR-78 spinel added MgO-C brick.
Hence, the above results of the micron and nano MgAl2O4 spinel added MgO-C bricks clearly show the potential application in the slag lines of ladle furnace. Keywords: MgO-C refractories; MgAl2O4; Nanopowders; Slag corrosion resistance; Oxidation resistance; Spinel.
i
ii
List of Figures Page No
Fig.1.1: Schematic view and various parts of steel ladle 04
Fig. 2.1: Different phenomena of corrosion in refractories 17 Fig. 2.2: Different penetration conditions of slag in refractory 18 Fig. 3.1: Schematic flow diagram for the preparation of MgAl2O4 spinel
nanopowder 27
Fig.3.2: Rotary furnace for conducting slag corrosion test for micron-sized spinel added MgO-C bricks
30
Fig. 4.1: HMOR and TSI as a function of different types and amounts of micron-spinel added MgO-C bricks
37
Fig. 4.2: Black surface remaining in % after oxidation resistance test for different bricks
38
Fig. 4.3: Surface pattern of different spinel type MgO-C bricks after slag corrosion test
39
Fig. 4.4: Corrosion (mm) as a function of different spinel added MgO-C refractories
39
Fig. 4.5: Optical micrographs of (a) normal and (b) large crystal of 97 % fused MgO
41
Fig. 4.6: Optical micrographs of MgO-C bricks without spinel addition after slag corrosion test which indicate (a) Crack formation and (b) disintegration of MgO grains
42
Fig.4.7: Optical micrograph shows graphite intact for AR-78 spinel added MgO-C bricks after slag corrosion test
43
Fig. 4.8: Optical micrographs of MgO-C bricks (a) without spinel and (b) with AR-78 spinel after slag corrosion test
44
Fig. 4.9: DSC-TG curve of the gel 45
Fig.4.10: XRD patterns of as-prepared spinel nanopowders calcined at different temperatures
46
Fig.4.11: SEM micrograph of MgAl2O4 nanopowder 47
Fig. 4.12: HMOR and TSI as a function of spinel added MgO-C refractory 49
Fig. 4.13: Black surface remaining in % as a function of spinel addition in MgO-C refractory
50
Fig. 4.14: Surface pattern of different spinel type MgO-C samples after slag corrosion test
51
Fig. 4.15: Corrosion (mm) as a function of spinel added MgO-C refractories 52
Fig. 4.16: Optical micrographs of (a) 0.5 % and (b) 1 % nano spinel added MgO-C refractories after slag corrosion test
54
iii
List of Tables Page No
Table 1.1: Different working lining designs in steel ladles in India 04
Table 2.1: Technological evoluation of MgO-C refractory 10
Table. 2.2: Chemical and physical properties of magnesia aggregate 12
Table 2.3: Characteristics of flake graphite used for carbon containing refractories
12
Table 2.4: Various routes for preparation of nano MgAl2O4 spinel 21
Table 3.1: Physical and chemical analysis of flake graphite 24
Table 3.2: Physical and chemical analysis of liquid resin and pitch powder 24
Table 3.3: Chemical composition in percentage of fused magnesia and spinel
25
Table 3.4: Batch composition of micron-sized spinel added MgO-C bricks 25
Table 3.5: Batch composition of nano spinel added MgO-C refractory 26
Table 3.6: Mixing sequence of MgO-C bricks 26
Table 3.7: Chemical composition (%) and basicity of the steel making ladle slag
30
Table 4.1: AP (before and after coking) of MgO-C refractories with addition of micron-sized spinel
34
Table 4.2: BD (before and after coking) of MgO-C refractories with addition of micron-sized spinel
34
Table 4.3: CCS (before and after coking) of MgO-C refractories with addition of micron-sized spinel
34
Table 4.4: Distribution of pores in MgO-C bricks after slag corrosion 40
Table 4.5: AP, BD and CCS of nano-sized MgAl2O4 spinel added MgO-C refractories, correlated with ZS and AR-78 added MgO-C refractories
48
Table 4.6: Distribution of pores in spinel added MgO-C refractories after slag corrosion
53
iv
1
Chapter 1
GENERAL INTRODUCTION
1.1 Introduction
Refractories play an important role in metallurgical, glassmaking and ceramic
industries, where they are formed into a variety of shapes to line the interiors of furnaces
or kilns or other devices for processing the materials at high temperatures [1-2]. Many of
the scientific and technological inventions and developments would not have been
possible without refractory materials. Dreaming about producing one kilogram of any
metal without the use of refractory is almost quite impossible. The ASTM C71 defines
the refractories as "nonmetallic materials having those chemical and physical properties
that make them applicable for structures or as components of systems that are exposed to
environments above 1000 °F (538 °C)" [3].
The type of refractories to be used is often dictated by the conditions prevailing in
the application area. Generally, refractories are classified into two different groups [4]:
(a) based on raw materials, the refractories are subdivided into three categories such as
acidic (zircon, fireclay and silica), basic (dolomite, magnesite, magnesia-carbon, chrome-
magnesite and magnesite-chrome) and neutral (alumina, chromite, silicon carbide, carbon
and mullite) and (b) based on manufacturing process, the refractories are subdivided into
two categories such as shaped refractories (available in the form of different brick shapes,
and includes the oxide and non-oxide systems) and unshaped refractories (which includes
mortars, castables and monolithic).
In tune with the changing trends in steelmaking, especially in ladle metallurgy,
the high performing shaped refractories are on an increasing demand in recent years. The
higher campaign lives and the variability of newer steel making operations are decided by
the availability and performance of such shaped refractories with superior high
temperature mechanical strength, erosion and corrosion resistance [5]. Initially, the ladles
were used only to transport the steel from steel making unit to casting bay, but now-a-
days the refining process is also carried out in the same. Thus, steel producers throughout
the world have been putting on a continuous effort to improve the ladle life in order to
increase the performance of ladles as well as reduce the specific consumption of
refractories so as to have a strong grip over cost and quality of steel and also to increase
the ladle availability with lesser number of ladles relining per day [6]. Due to the above
2
reasons, there had been a great technological evolution in ladle lining concept
such as: zonal lining concept, which deals with both selection of refractory quality and
refractory lining thickness [7-10]. In today’s scenario, it is quite impossible to imagine a
steel teeming ladle without magnesia-carbon (MgO-C) refractory bricks. MgO-C bricks
have dominated the slag line of ladles for at least a decade as they possess superior slag
penetration resistance and excellent thermal shock resistance at elevated temperature
because of the non-wetting property of carbon (graphite) with slag, high thermal
conductivity, low thermal expansion and high toughness [11, 12]. However, the life of
these refractories has become somewhat limited on prolonged use and increasing severity
of operating condition due to its poor oxidation resistance as well as low strength at high
temperatures [13]. The lining of ladle depends to a greater extent on the wear rate of
MgO-C refractory arising from slag penetration and structural spalling. Increased steel
production has led both refractory manufacturers and users to resume interest on further
improvement of thermo-chemical properties of MgO-C refractories [14]. In recent years,
magnesium aluminate (MgAl2O4) spinel has been recognized as one of the most effective
refractory material due to its excellent wear and penetration resistance towards slag and
also resistance to change in operating environment [15-18]. Presence of micron sized
MgAl2O4 spinel as well as in-situ spinel (formation took place in the matrix by the direct
reaction of magnesia and alumina) in the matrix of MgO-C refractories improves the
thermal shock resistance and corrosion behavior of refractory products [18]. Presence of
nano (size < 100nm) particles in MgO-C refractories have also improved the durability,
thermal shock resistance, corrosion resistance and oxidation resistance [19-21]. Thus it is
interesting to study the physical and chemical properties of MgO-C bricks with the
addition of micron-sized and nano-sized MgAl2O4 spinel.
Hence, in this chapter, a general introduction to MgO-C refractories and its
application in ladles, role of spinel in MgO-C refractory and role of nanoparticles in
refractory industries are described based on literature. The organization of thesis is given
in the last part of this chapter. The main objectives of the present research work are
presented at the end of the second chapter, which is based on a critical literature survey.
3
4
Fig.1.1: Schematic view and various parts of steel ladle
1.2 MgO-C refractory and its application in ladle
Refractories used for ladle lining must able to withstand the increasing severity of
service conditions associated with secondary steel making in order to produce various
grades of steel with stringent specifications. The condition during the steel refining
processes are aggressive, which makes the refractory materials used in steel teeming ladle
susceptible to high degree of corrosion. In addition to corrosion, brittle nature of
refractory materials gives limitation to their applicability. Fig 1.1 shows the schematic
view and various parts of the steel ladle. The different working lining designs of the steel
ladle is given in Table 1.1.
Table 1.1: Different working lining designs in steel ladles in India
Area Bottom Metal Zone Slag Zone Free Board
Refractory bricks used
MgO-C Al2O3-MgO-C 70% Al2O3 80% Al2O3 MgO-Cr2O3
MgO-C Dolomite Al2O3-MgO-C 70% Al2O3 80% Al2O3 Cr2O3-MgO
MgO-C 70% Al2O3 80% Al2O3 MgO-Cr2O3
MgO-C 70% Al2O3 80% Al2O3 Cr2O3-MgO
5
Some of the important properties requirements of refractories used in steel ladle are:
• High corrosion resistance to steel slag
• High abrasion resistance by liquid metal
• High thermal spalling resistance
• High hot strength and
• Low molten steel penetration
For the past several years, refractories based on MgO and C had performed
tremendously well in many applications such as basic oxygen furnace (BOF), electric arc
furnace (EAF), varieties of vessels and ladles for secondary refining treatments as
compared to bricks without carbon due to high thermal conductivity, low thermal
expansion, chemical inertness to slag and high thermal shock resistance [1-2].
MgO-C refractory, which is one of the highest consumable refractory item in steel
sector with a specific consumption as high as 3.0 kg/ton in BOF and 2.5 kg/ton in EAF
for the best shop’s practice is the top most concern for any steel manufacturer. MgO-C
refractories are unfired refractory, which is manufactured by mixing refractory grains,
graphite and other additives with liquid resin and pitch as a binder and uniaxially pressed
using a hydraulic press with a specific pressure of 2 T/cm2. The pressed bricks were
tempered at 220-240 °C, to facilitate polymerization of resin into carbon and to eliminate
residual water and phenols, there by developing sufficient strength [22]. The physical,
thermo-mechanical and thermo-chemical properties of MgO-C refractories have
improved significantly by selecting the right raw materials with respect to purity, grain
size of MgO, binders, bonding systems and additives in both micron and nano range [5,
11, 12, 22, 23].
1.3 Role of spinel in MgO-C refractory
The spinel minerals have the generic formula AB2O4, where ‘A’ is a divalent ions
such as Mg2+, Fe2+, Mn2+, Zn2+ and ‘B’ is a trivalent ions such as Al3+, Fe3+. The structure
of spinels was described as having an oxygen ion sub lattice arranged in a cubic close-
packed arrangement with cations occupying various combinations of the octahedral (O)
and tetrahedral (T) sites. The cubic unit cell is large, comprising 8 formula units and
containing 32 O and 64 T sites. Spinels are divided into two categories such as normal
6
and inverse spinel. In normal spinel, the divalent cations ‘A’ are located on the
tetrahedral (T) sites and the trivalent cations ‘B’ on the octahedral (O) sites. In inverse
spinels, the A cations and one-half the B cations occupy the O sites, with the remaining B
cations occupying the T sites [24].
MgAl2O4 spinel ceramic is of significant technological interest for refractory and
structural applications at elevated temperature because spinel (MgAl2O4) is a refractory
material, where no liquid formation takes place with any mixture of pure magnesia and
alumina at temperature below 1900 °C. It has also high melting point, good mechanical
strength and excellent chemical resistance. The major application areas of spinel
refractories are transition and burning zones of cement rotary kilns, sidewalls and bottom
of steel teeming ladles and checker work of glass tank furnace generators because they
are resistant to corrosion by slag [25-29]. For such applications, spinel is used as a major
component in an alumina rich or magnesia rich matrix, depending upon the
environmental condition prevailing in the application zone. Hence, stoichiometric,
magnesia rich and alumina rich spinel (non-stoichiometric) compositions are important
from the application point of view.
Spinel always have a tendency for forming substitutional solid solution when
comes in contact with slag due to its defective structure [30]. A complex nature of spinel
such as (Mg, Mn, Fe)O·(Fe, Al)2O3 was formed when Fe2+ and Mn2+ of the slag goes into
A-site of spinel. Also Ca2+ of slag reacts with excess Al2O3 of spinel forming Hibonite
(CA6) leading to densification of texture [30, 31]. Depletion of MnO, FeO and CaO
makes the slag more viscous (due to increase of the relative amount of SiO2), which
limits slag penetration and thereby reduces slag corrosion [32].
1.4 Role of ceramic nanoparticles in refractory industry
The refractory industry is highly matured and in order to counteract the stiff
competition from foreign market, the only way is to develop new technologies that have
high added value and cannot be easily copied. Thus the use of nanoparticles has brought
about a revolution in refractories field by exhibiting remarkable performance [19-21].
Nanoparticles are nothing but ultrafine particles of size < 100 nm. When the grain size of
the material reduces to nano scale, the relative volume of atoms in the grain boundary
7
enhances and the ordered arrangement conditions of original atoms or molecules will be
destroyed leading to alteration of many properties such as structural, microstructural,
chemical and mechanical [33, 34]. A small amount of nanoparticle addition in
refractories has a great influence on its thermo-chemical properties. Nanoparticles
disperse among spaces between coarse, medium and fine particles of refractory raw
materials thereby filling of interior pores and gaps and improve the microstructure and
reactivity [21]. Nano materials not only absorb and relieve the stress due to thermal
expansion and shrinkage of refractory particles but also reduce the maldistribution of
thermal stress in the inner portion of refractories [21]. Incorporation of nano materials
also increases the strength and corrosion resistance of refractory at high temperature due
to its high surface to volume ratio [21].
Addition of small amounts (~ 2 wt %) of nano-zirconia (ZrO2) in dolomite
refractories resulted in the improvement of densification, thermal shock resistance,
slaking resistance and slag corrosion resistance [35]. Presence of nano iron oxide in
MgO-Cr2O3 refractories facilitated the formation of magnesio ferrite spinel at lower
temperatures which improves the physical and chemical properties of the bricks [36].
Addition of 0.4 wt% nano Fe2O3 in silica refractories has improved the physical and
chemical properties [37].
The castables used in iron and slag runners in blast furnace possesses superior
slag corrosion resistance, excellent thermal shock resistance and mechanical properties
due to the formation of nano-sized SiC whiskers (additives present in the matrix such as
Si and FeSi2 results in formation of nano sized SiC whiskers at 1400 °C) [38]. A
developed technique to study the hydration of castables was based on measuring the
electrical conductivity. Addition of nano-sized poly carboxylate-ether based
deflocculants lowers the electrical conductivity of the matrix suspension to values near
0.71 ms/cm there by facilitating achievement of self flowabilty of the castable [39].
Addition of nano MgAl2O4 gel in castable system has resulted in tremendous
improvement in thermal shock and corrosion resistance as compared to micron sized
spinel addition [40-42].
8
1.5 Organization of the thesis
The addition of micron or nano ceramic in MgO-C refractories has significantly
improved the thermo-chemical properties. Basic introduction of MgO-C refractories and
its application in ladle along with the role of spinel and nano ceramics in refractories was
discussed in chapter 1. Chapter 2 provides a detailed discussion of literature on different
works on MgO-C refractories with respect to various types of raw materials, additives
and binders. It also deals with literature review on synthesis of nano crystalline spinel
through various non-conventional routes. It also covers the effect of physical and
chemical properties of MgO-C refractory with the addition of nano materials. The main
objective of the present work, which is based on the literature survey, is presented
towards the end of chapter 2. Chapter 3 deals with the raw materials and refractory
fabrication along with synthesis of nano-sized spinel using citrate-nitrate route. The
characterization techniques used in the present work are described in detail in this
chapter. Chapter 4 deals with the study of physical and chemical properties of micron-
sized spinel addition in MgO-C refractories with respect to type and amount;
characterization of nano MgAl2O4 spinel powders synthesized using citrate-nitrate route
and the effect of nano MgAl2O4 spinel addition on the physical and chemical properties
of MgO-C refractories. Finally, conclusions and scope for the future work are given in
Chapter 5.
9
Chapter 2
LITERATURE REVIEW
10
2.1 Technological evolution of MgO-C refractories
Since 1950’s, carbon has been recognized as an essential component of
refractories. It was found that the addition of carbon leads to better thermal and chemical
resistance, thereby increasing the life of refractory linings and indirectly reducing steel
production cost [43, 44]. Carbon is now an integral component of the ceramic-carbon
composite for many refractory applications. State-of-the-art, magnesia-carbon brick is
the accepted standard for lining BOF and electric steelmaking furnaces and for the slag
lines of ladle metallurgy furnaces [45]. The detail technological evolution of MgO-C
refractories and its application area is given in Table 2.1.
Table 2.1: Technological evolution of MgO-C refractory [46, 47]
Year Technology Evaluation
1950
• Evolution and use of magnesia carbon and pitch bonded dolomite refractories; carbonisation carried out during preheat treatment of ladle; inhibiting slag penetration and thermal spalling.
• Used in BOF.
1970
• Magnesia purity became a factor. Thus MgO grain with low boron and lime to silica ratio of 2 to 3:1 was used extensively to improve corrosion resistance.
• Burned and impregnated magnesia brick with finite pore size to inhibit slag penetration and thermal spalling
• Used in charge pad and other high wear areas in BOF. • Beginning of zonal lining concept.
1980
• Development of resin bonded magnesia-graphite refractories with higher carbon content.
• Addition of antioxidants to preserve the carbon content.
2000 – Till date
• Use of high purity magnesia grains (fused / sintered) having large crystal size to further improves the corrosion resistance.
• Variation of carbon content with respect to type and amount to improve the thermal conductivity and oxidation resistance.
• Addition of various additives (such as metallic, alloy and inorganic compounds) to achieve improved hot strength, oxidation resistance and corrosion resistance.
• In-situ spinel bonding to improve thermal spalling. • Use of nano additives.
11
In spite of several efforts made to improve the performance of MgO-C bricks, the
problems still exist due to increasing severity of operating condition by many folds. This
has opened up the path for further research in this field. This is how use of spinel in
refractories has come up in a broad way. Inconsistency in performance due to
inhomogeneous microstructure has led several researchers to think for some alternative
methods to achieve the desired properties and a consistent performance which has led to
explore the possibility to incorporate nano additive in the matrix [19, 48]. The selection
of base raw material greatly influences the properties and performance of refractories and
was discussed in detail.
2.2 Selection of raw materials
The main problems faced in steel ladle refractories are corrosion by steel slags,
abrasion by liquid metal, thermal spalling, oxidation of carbon layer, deterioration of
strength at high temperature and molten steel penetration [49-51]. The performance of
refractories greatly depends on the selection of raw materials. Several studies had been
carried out to find out the effect of different raw materials based on purity, porosity and
crystallite size [52-54]. The raw materials include magnesia, graphite, resin and
antioxidants. Selections of individual raw materials are described in detail.
(a) Magnesia
Three different types of magnesia grains are used for the production of MgO-C
bricks such as - sintered magnesia produced from natural magnesite; seawater magnesia
produced by firing magnesium hydroxide extracted from seawater and fused magnesia
produced by fusing sintered magnesia in an electric furnace [55, 56].
Several researchers reported the effects of magnesia aggregate on the corrosion
resistance of MgO-C bricks. It was indicated that the magnesia aggregate with following
characteristics, which led to superior corrosion resistance.
(i) High concentration of fused magnesia rather than sintered magnesia [53, 57].
(ii) Small content of B2O3 and high ratio of CaO/SiO2 [58-60]. (iii) Large periclase crystal grain [58]
12
The typical chemical and physical properties of magnesia aggregate are given in Table 2.2.
Table. 2.2: Chemical and physical properties of magnesia aggregate [58, 59].
Properties Products
Seawater Natural Brine Fused Sintered Fused Sintered Sintered
Chemical composition (%)
MgO SiO2 Al2O3 Fe2O3 CaO B2O3
99.07 0.20 0.06 0.11 0.57 0.02
99.13 0.22 0.06 0.04 0.51 0.04
96.55 1.29 0.12 0.75 1.19
Traces
98.32 0.57 0.08 0.44 0.58
Traces
95.46 1.96 0.90 0.67 0.98
Traces
99.30 0.02 0.05 0.01 0.67
Traces
Apparent porosity (%) 2.60 1.50 1.10 0.80 8.0 2.0
Bulk specific gravity 3.46 3.40 3.54 3.55 3.20 3.43
Periclase grain (µm) >200 20-40 >50 >100 20-60 20-40
(b) Graphite
Carbon in the form of natural-flake graphite made up of well-formed crystals was
often used in MgO-C brick. Characteristics of flake graphite are given in Table 2.3.
+++ Very strong ++ Strong + Weak - Very weak
Table 2.3:Characteristics of flake graphite used for carbon containing refractories [13, 61]
Characteristics Source
China India Malaysia Japan 1 2 Grain size distribution (wt-%)
> 0.5 mm 0.50 - 0.297 mm 0.297 - 0.177 mm 0.177 - 0.125 mm 0.125 - 0.063 mm < 0.063 mm
13.2 35.0 26.4 11.4 8.9 5.0
0.9 8.8 49.1 38.3 2.4 0.4
2.6 28.2 65.6 3.0 0.4 0.2
12.5 27.3 40.6 13.2 5.2 1.2
- -
6.8 12.7 42.5 38.0
Avg. grain size (mm) 0.286 0.194 0.262 0.276 0.075
Ash content (%) 14.5 6.0 12.6 11.8 13.7
Minerals in ash: Quartz Mica (Biotite) Kaolinite Chlorite Feldspar Vermiculite
+ + + + - +
+ + + - + +
+++ ++ + - + +
+
+++ ++ - - +
++ - + + + +
13
Presence of minerals like quartz, kaolinite and anorthite in ash of graphite
possesses an adverse effect on the corrosion resistance of MgO-C brick. Impurities in ash
of flake graphite after decomposition reacts with MgO grains to form low melting phases,
thereby decreases the corrosion resistance [62]. Hence, carbon purity should be kept as
high as possible. The roles of graphite are (i) it fills the porous brick structure; (ii) hinders
the slag penetration in to the brick due to high wetting angle between slag and graphite
that leads to the formation of dense layer of MgO and CO at the slag-brick interface and
(iii) improves the thermo-mechanical spalling (surface splitting of the lining) resistance
of brick due to high thermal conductivity and low thermal expansion of graphite. The size
of graphite also plays a vital role for improving the oxidation, abrasion and corrosion
resistance of MgO-C bricks [63].
The major problem faced during manufacturing MgO-C brick is compressibility
of graphite in the mixture to get a dense structure. Thus pressing of a dense brick greatly
depends on the type of binder used.
(c) Resin
Initially, pitch was used as binder for MgO-C brick. However, it was difficult to
prepare a dense brick containing a large amount of flake graphite due to the elastic
character of graphite, which causes the brick to expand during heat treatment leading to
poor adhesion of graphite to the matrix. Hence resin was found to be the best binding
agent for MgO-C refractories [64].
Phenolic resin is the most common binder used in carbon containing refractories due to
the following excellent features.
(i) Chemical affinity towards graphite and refractory aggregates
(ii) High adhesive property leading to high handling strength.
(iii) Being thermosetting in nature it imparts high dry strength.
(iv) Strong carbon bonding was achieved due to high content of fixed carbon (52%).
(v) Environmentally, it was less harmful than tar pitch.
(vi) Superior kneading and pressing characteristics.
(vii) Polymerization of resin (100-200°C) leads to isotropic interlocking structure.
(viii) Higher resin content increases the cold crushing strength (CCS) and strength of
the tempered bricks.
14
During winter, the viscosity of resol resin increases, which often causes low
dispersion of ingredients in the mixer machine [65]. On the contrary, in summer, the
viscosity of resin sometime causes the green body to weaken its stiffness, resulting in
lamination of bricks [65]. In order to overcome the reduction in viscosity, powder
novalac resin was added into resol resin [65].
Main demerit of carbon bearing material is the removal of carbon through
oxidation at high temperature. This process makes the brick texture loose and prone to
attack by slag thereby reducing the life of the refractory brick [66]. Thus to check the
removal of carbon by oxidation, metallic addition was done in smaller amounts which
was known as antioxidants.
(d) Antioxidants
The main drawback of carbon containing refractories was the oxidization of
carbon. The oxidation of carbon took place in two different ways [67, 68]: direct
oxidation and indirect oxidation. Below 1400 °C, direct oxidation occurs when carbon
was oxidized directly by the oxygen from atmosphere. Above 1400 °C, indirect oxidation
took place that leads to a partial loss of both Mg and C from the refractories. On
prolonged exposure to temperature above 1500 °C, Mg vapor forms and simultaneously
deoxidizes to MgO. A dense secondary oxide phase of MgO layer adjacent to the hot face
of the refractories was formed, that causes an increase in oxidation resistance of the
material during operation at high temperature. Thus to prevent oxidation of carbon,
different antioxidants such as aluminium (Al), silicon (Si) and boron carbide (B4C) are
used in MgO-C refractories [66, 68-72]. Al and Si antioxidants are mostly used due to
their low cost and effective protection, which once formed remain stable as a discrete
phase in the bulk of the specimen. The formation of Al4C3 and SiC inhibits the oxidation
of carbon [68]. B4C reacts with air to form liquid boron oxide, which adheres to the
refractory surface as a protective layer thus preventing oxygen to come in contact with
refractory material [69, 71]. Now-a-days, new generation of boron based antioxidants like
ZrB2, CaB2, CaB6, Al8B4C7, Mg-B, CrB2 and SiB6 have come to the market that react to
form liquid phase, thereby filling the pores and preventing the oxidation of carbon [73-
81].
15
2.3 Role of micron-sized, stoichiometric and in-situ spinel in MgO-C brick
In recent years, MgAl2O4 spinel is of significant technological interest for
refractory applications at elevated temperature as because it is an environment friendly
material possessing and also a good combination of both physical and chemical
properties [16, 82-86] such as:
• High-melting point • High chemical inertness against both acidic and basic slags • Low thermal expansion at elevated temperatures • Resistance to slag corrosion • High thermal shock resistance • Excellent hot strength • Low content of secondary oxide phases, providing good refractoriness • High resistance to changes in the environment and • Ecologically benign refractory material
MgAl2O4 spinel added MgO-C refractory has been improved continuously under
ecological and economical aspects, mainly in terms of binders and additives used for
better thermo-mechanical properties and reinforced oxidation resistance [27]. In addition
to that, MgAl2O4 spinel added MgO-C refractories exhibit unique mechanical, thermal
and chemical properties. Therefore they are established as high duty refractory products
in application of various parts of converters, slag zone of electric arc furnaces and ladles
[25-27].
Various grades and sizes of MgAl2O4 spinels are commercially available in the
market with different alumina and magnesia contents. Depending upon the application
condition, the type of spinel was chosen. MgO rich spinel addition in refractories is
preferred for cement rotary kilns, whereas refractories containing Al2O3 rich spinel are
preferred for steel ladles [82, 83, 87, 88]. It was observed that high alumina castables
with micron sized spinel addition have given superior performance in the sidewalls and
bottom of steel ladles along with MgO–C bricks in slag line due to depletion of MnO,
FeO and CaO in slag and the formation of CA6 which make the slag more viscous and
less penetrative [89, 90]. Addition of micron-sized spinel in the refractories increases the
slag corrosion resistance [90, 92].
16
It was reported that addition of stoichiometric spinel improves the slag erosion
and penetration resistance due to the formation of gehlenite (C2AS), CA2 and CA6 phases
at the hot face [15]. So, the relative amount of silica increases to generate a high viscous
and high melting temperature slag which may be a probable cause for preventing further
slag penetration resulting in improved slag resistance for spinel added high alumina
castables [89].
The properties of refractory materials can also be enhanced by in-situ spinel
formation in the site. The in-situ spinel formation starts around 1000°C and gets
accomplished above 1300°C [17, 18]. It is accompanied with volume expansion which
leads to a significant reduction in pore volume [16, 93-95]. The formed spinel particles
are found almost in the periphery of the periclase grains and play a vital role in improving
the refractory properties. Formed spinel minimizes the open pores and leads to
densification of matrix thereby preventing slag penetration. In-situ spinel formation also
improves the corrosion and thermal shock resistance [16, 17, 96, 97]
The amount of in-situ spinel formation was to be optimized to get sufficient
tightening of the joints, which prevents the liquid metal penetration. Structural spalling
resistance was increased due to the development of micro cracks [mismatch thermal
expansion co-efficient between MgO (13.5 X 10-6/°C) and MgAl2O4 (7.6 X 10-6/°C)
grains] [16, 17]. On other hand, higher amount of spinel formation leads to higher
expansion and thereby leading to development of stresses, which causes structural
spalling and increased slag penetration [16, 17]. So, controlled spinel formation is always
desirable.
17
2.4 Mechanisms of corrosion in MgO-C bricks
During refining of steel in ladle, corrosion of the lining material in contact with
slag took place due to the following phenomena [90, 98, 99]. Fig. 2.1 shows the different
phenomena of corrosion in refractories.
Fig. 2.1: Different phenomena of corrosion in refractories [Adapted from ref. 98, 99]
(i) Dissolution is a chemical process by which the refractory material was
continuously dissolved by the diffusion of reacting species through the liquid slag.
(ii) Penetration is a process by which the slag penetrates into the pores that causes
deterioration of the refractory wall due to differential expansion or contraction
between refractory and the slag.
18
(iii) Erosion is the process of wear out of refractory material which depends on
viscosity of slag and velocity of gases that comes in contact with the refractory
material.
Corrosion of carbon containing refractories follows the following three stages
simultaneously with the above phenomena [98-100] such as:
(i) Formation of a decarburized layer that may be due to oxidation of graphite.
(ii) Infiltration of slag into the decarburized layer and erosion of the oxide grain.
(iii) Reduction of oxide grains at high temperature (~1600°C) reaction with carbon
those results in its exposure to slag and further erosion.
Diffusion of slag particles into refractory material causes a change in the physical
properties. The higher wetting angle makes it more difficult for the slag to penetrate into
pores and cracks in the refractory [101]. This was not the only thing that affects the
infiltrating depth. The infiltrating depth was also affected by the temperature gradient in
the brick [101]. The temperature gradients causes the viscosity of slag to increase with an
increasing distance into the refractory (colder), thereby decreasing the infiltration depth
will get decrease. Fig. 2.2 shows the penetration of slag in the refractory as it proceeds
from hot face to cold face. The penetration depth depends on hot face temperature of
refractories, slag temperature and its viscosity. Penetration increases with increase in hot
face temperature, slag temperature and decrease in slag viscosity.
Fig. 2.2: Different penetration conditions of slag in refractory. [Adapted from ref. 98]
19
2.5 Effect of nanoparticles on the properties of MgO-C refractories
Nano technology has been introduced into refractories field in recent years in
order to eliminate the problems related with their performance arising out of
inhomogeneous microstructure. It has been apprehended that the performance of
refractories could be appreciably improved by improving the thermo-chemical properties
due to well dispersion of nano-sized particles in the matrix of the refractories [33, 34].
The refractory brick is made up of aggregate and matrix. The aggregate part is
composed of particles of size ranging from several micrometers to millimeter. The matrix
part is composed of particles of size less than or equal to 500 µm. Around 25 vol % of the
total brick structure was occupied by the matrix. Out of which 10 vol% comprises of
pores. The physical and chemical properties of the refractories depend on the particle
size, pore size as well as its distribution and gap between aggregate and matrix phase
[19]. Thus nanoparticles can easily modify the microstructure as per the requirements by
filling the gap and modifying the pore size distribution.
Use of nearly 5 vol % (~1.5 wt-%) nano carbon (two different types such as single
sphere and aggregate) in MgO-C refractory has improved the thermal shock resistance,
and bond strength [19]. The addition of single sphere type nano carbon has led to
densification of matrix, thereby improving the erosion resistance. Aggregate type of nano
carbon provides elasticity, which in turn decreases the stress relaxation and improves the
thermal shock resistance. In addition to this, aggregate type nano carbon provides pore
segmentalization and pore volume control (rich in micro pores), thereby leading to
minimization of heat loss and avoiding shell deformation. However, the combination of
both types of nano carbon in MgO-C refractory counter balances the thermal spalling
resistance and corrosion resistance [19].
Use of low amount of nano carbon (2 vol %, 10 nm size) and higher amount of
flake graphite (8%, ~ 0.3 mm) in MgO-C refractories improves heat insulation and
decreases the shell deformation and increases shell life of vessel [21]. It was also reported
that the addition of 1.5% nano-particles showed better thermal spalling resistance as
compared to that of refractories containing 18% graphite [20].
20
Titanium carbide is an excellent non oxide ceramics with high melting point,
hardness and electrical conductivity with good wear resistance, corrosion resistance,
thermal conductivity and good chemical stability. However, the use of titanium carbide in
refractory industry was limited because of its high cost. Recently, Arasu et. al. [102] has
investigated the formation of in-situ titanium carbide in the matrix of the MgO-C system
by adding nano TiO2 that improves the physical and chemical properties of MgO-C
bricks.
2.6 Synthesis of MgAl2O4 spinel using different chemical routes
MgAl2O4 spinel is industrially produced either from magnesia and alumina or
magnesite and bauxite by fusion or sintering. Spinel aggregate produced by fusion or
sintering routes have relatively low reactivity. Different synthesis routes have been
developed for producing MgAl2O4 spinel [103-109]. Different additives were introduced
for betterment of physical and chemical properties of MgAl2O4 spinel produced which
will be used in the refractory products [110-112]. It was very difficult to produce ultra
fine, reactive spinel powders from the aggregates. For this reason, various wet chemical
methods have been successfully developed for producing nano spinel powders [113,
114]. Table 2.4 shows the different processes implemented by various researchers using
different processing conditions.
The precursor particles produced through different wet chemical routes tend to
agglomerate during drying. Severely agglomerated spinel powders have difficulty in
sintering, especially at relatively low temperatures. Therefore combustible ingredients are
introduced into the precursors prepared by co precipitation to reduce the formation of
hard agglomerates during drying and firing. Hence, in this work, MgAl2O4 spinel
nanopowders have been synthesized through citrate-nitrate route.
21
Table 2.4: Various routes for preparation of nano MgAl2O4 spinel
Methods Remarks References
Citrate-nitrate Citrate to nitrate ratio 1:1; MgAl2O4 formation started at 650 °C and size was 30 – 50 nm.
[115, 116]
Co-melting 1:1 to 1:1.4 ratio of Al to Mg nitrates; crystallite size was 12-59 nm.
[117]
Co-precipitation 1:2 ratio of Mg and Al with sintering aid (ZnO or MnO2). pH maintained between 9.5-10.5 Particle size was 25-60 nm.
[118]
Sol-gel Metal alkoxides of Al(OC3H7)3 and Mg(OC2H5)2 were used. Surface area of amorphous powder is 260 m2/gm. Crystallite size was 30 nm.
[119]
Sol-gel citrate Spinel (size around 20 nm) formation started at 400°C
[120]
Microwave assisted
combustion
Use of modified domestic microwave oven Crystallite size of spinel synthesized using microwave and combustion synthesis was 20-50 nm and 100-250 nm, respectively
[121]
Freeze drying Production of fine homogeneous particles. Particle size of spinel powder after calcined at 1100°C/12 h is about 50 nm.
[122]
Flame spray pyrolysis
Resultant spinel powder was spherical, dense and homogeneous. Specific surface area is 40-60 m2/g. Average particle size is 25-45 nm.
[123]
2.7 Summary of literature
The extensive literature survey reveals that in spite of several research regarding
the improvement of life and performance of MgO-C refractories with respect to different
types of raw materials (type, crystalline size and purity), binders (type and viscosity) and
additives (carbon, antioxidants and special oxides), still there is a scope of further
improvement on the properties and performance of MgO-C refractories due to increase in
severity of operating condition, greater demand for production of cleaner steel and low
specific consumption of refractory in steel sector.
22
MgO-C bricks were used in slag line of ladles due to superior slag penetration
resistance and excellent thermal shock resistance. The life of this refractory has limited
on prolonged use and increasing severity of operating conditions due to poor oxidation
resistance and low strength at high temperatures. It was observed from the literature that,
addition of MgAl2O4 spinel (either in micron, or stoichiometric or in-situ) exhibits unique
mechanical, thermal and chemical properties of refractories.
The particle size of spinel is also an important factor that influences both physical
and chemical properties of refractories. Addition of nanoparticles in different refractory
systems has resulted in tremendous improvement in thermo-mechanical as well as
thermo-chemical properties. A very few literatures are available on the effect of the
physical and chemical properties of MgO-C bricks with addition of micron-sized and
nano-sized MgAl2O4 spinel. Thus, there is further scope to improve the thermo-
mechanical as well as thermo-chemical properties of MgO-C refractories with addition of
micron-sized (type and amount) and nano-sized MgAl2O4 spinel.
2.8 Objectives of the present studies
The main objective of the present work:
To improve the physical and chemical properties of MgO-C refractories with the
addition of MgAl2O4 spinel in micron (with respect to type and amount) and nano range.
In this work, a set of experiments was carried out in order to standardize the type
and amount of preformed spinel addition in MgO-C refractory system. Here, micron-
sized spinel in three different commercially available grades [near stoichiometric (AR-
78), alumina rich (AR-90) and magnesia rich (MR-66)] were used.
The standardized type and amount of spinel (10 wt % AR-78) was taken in order
to compare and carry out the second set of experiments. In this experiment, the effect of
without, standardized micron-sized (10 wt % AR78) and nano-sized MgAl2O4 spinel
added MgO-C bricks are correlated.
The spinel nanopowders were prepared by citrate-nitrate route, which is known to
result in the production of nanocrystalline materials and these nanopowders (calcined at
800 °C) were used during fabrication of nano spinel added MgO-C brick.
23
Chapter 3
EXPERIMENTAL WORK
This chapter covers three sections. The first section covers the properties of
different types of raw materials used for the fabrication of micron and nano-sized spinel
added MgO-C brick. Second section covers the synthesis procedure for the preparation of
nano-sized MgAl2O4 spinel and third section describes the different characterization
technique to study the different properties of micron-sized and nano-sized MgAl2O4
added MgO-C bricks.
24
3.1 Raw materials and fabrication of micron and nano spinel added MgO-C
brick
Commercially available high quality fused magnesia (FM), natural flakes
graphite, aluminium metal powder (–150 µm), three different types of micron sized
spinel such as AR-78, AR-90 and MR-66 with two different grading (- 45 µm up to 10
wt-% and 0.5 - 1.0 mm for 15 wt-% to 25 wt %) to maintain granulometry of the mixture.
Liquid resin and other additives were also taken as base raw materials for fabrication of
micron sized spinel added MgO-C bricks.
In this present work, FM97LC was selected as a raw material for fused magnesia
in order to get the purest MgO, best CaO/SiO2 ratio, lowest possible Fe2O3 content,
highest specific gravity and large crystals in the range of 500 – 1500 µm having less
number of grain boundaries [58, 59, 124]. The physical and chemical analysis of flake
graphite is given in Table 3.1 and the physical and chemical analysis of liquid resin and
pitch powder is given in Table 3.2.
Table 3.1: Physical and chemical analysis of flake graphite
Table 3.2: Physical and chemical analysis of liquid resin and pitch powder
Property Liquid Resin Pitch Powder
Viscosity (cps) Specific gravity Non-volatile matter (%) Fixed carbon (%) Moisture (%) Volatile matter (%) Ash (%) Softening point (°C)
8500 - 9000 1.23 80.10 47.85 ~ 4.0
- - -
- - -
52 -
47 1.4 135
Raw materials Carbon (%) Volatile matter (%) Ash (%) Surface area (m2g-1)
Flake graphite 94.1 1 5.3 1
25
The chemical composition of fused magnesia and three different types of spinel is
given in Table 3.3.
Table 3.3: Chemical composition in percentage of fused magnesia and spinel
MgO-C bricks with sixteen compositions have been fabricated using different raw
materials by varying micron sized spinel type and content starting from 0 to 25 wt % with
an incremental addition of 5 wt % in MgO-C refractories. In this work, 0 wt%
corresponds to without spinel added MgO-C brick and was denoted as ‘ZS’. The batch
composition (total 16 numbers) of MgO-C brick with micron sized spinel is given in
Table 3.4.
Table 3.4: Batch composition of micron sized spinel added MgO-C bricks
Raw materials Weight percentage
FM 97%LC 86.5 81.5 76.5 71.5 66.5 61.5 Graphite 12.5 12.5 12.5 12.5 12.5 12.5 AR-78 / AR-90 / MR-66 0 5 10 15 20 25 Al- metal powder 1 1 1 1 1 1 Resin liquid 3 3 3 3 3 3 Pitch powder 1 1 1 1 1 1
Raw Materials
Chemical composition
MgO Al2O3 SiO2 CaO Fe2O3 Na2O
Fused magnesia 97.34 0.08 0.40 1.40 0.50 0.50
AR-78 23.00 76.00 0.06 0.30 0.10 0.15
AR-90 9.00 90.00 0.05 0.25 0.10 0.17
MR-66 33.00 66.00 0.09 0.40 0.10 0.05
26
Nano spinel has been prepared using citrate-nitrate method (discussed in section
3.2). The as-synthesized spinel was calcined at 800 °C to get a pure MgAl2O4 spinel. The
calcined spinel nanopowders in different weight percentage such as 0.1, 0.5, 1 and 1.5
were used in MgO-C bricks. In this work, nano-spinel addition in MgO-C brick was
denoted as ‘NS’. The batch composition of nano spinel added MgO-C brick is given in
Table 3.5.
Table 3.5: Batch composition of nano spinel added MgO-C refractory
Raw materials Weight percentage
FM 97%LC 86.5 76.5 86.5 86.5 86.5 86.5 Graphite 12.5 12.5 12.5 12.5 12.5 12.5 AR-78 - 10 - - - - NS - - 0.1 0.5 1.0 1.5 Al- metal powder 1 1 1 1 1 1 Resin liquid 3 3 3 3 3 3 Pitch powder 1 1 1 1 1 1
All the raw materials were properly mixed thoroughly using high intensive mixer
machine at room temperature for nearly 40 minutes. Table 3.6 shows the mixing
sequence of various raw materials.
Table 3.6: Mixing sequence of MgO-C bricks
Steps Mixing Sequence Mixing Time (Min)
1 Coarse + Graphite + Aluminum metal powder + Hard pitch powder 2
2 Addition of liquid resin 15
3 Addition of dust fractions 20
4 Addition of resin powder 2
Total mixing time ~40
27
After mixing, micron sized spinel added bricks were pressed with a specific
pressure of 2 Ton / cm2 using hydraulic press (SACMI, Italy). Nano spinel added bricks
were pressed into cylinder of dimension 50 mm x 50 mm using laboratory uniaxial press
(1.8 Ton/cm2) rather than industrial press due to the non-availability of bulk amount of
synthesized spinel. Lot of difficulties were faced during mixing and pressing such as
improper dispersion while mixing and lamination as well as crack formation during
pressing for 1.5 wt % nano spinel addition. The pressed samples were tempered at 220-
250°C in a tempering kiln. Coking was carried out at 1000°C for 4 h under reducing
atmosphere (carbon bed). The physical and chemical properties of the micron as well as
nano-sized spinel added MgO-C bricks were characterized using different instrumental
techniques (discussed in section 3.3).
3.2 Synthesis of MgAl2O4 spinel nanopowders
Nanopowders of MgAl2O4 spinel were prepared using citrate-nitrate method. Fig.
3.1 shows the schematic flow diagram for the synthesis of nano MgAl2O4 spinel through
citrate-nitrate method.
Fig. 3.1: Schematic flow diagram for the preparation of MgAl2O4 spinel nanopowder
Mg(NO)3.6H2O Al(NO)3.9H2O
C6H8O7.H2O NH4OH
Mixing
Heating at 80 °C
Viscous gel
Heating at 80 °C to get blackish powder
Calcined at different temperatures
Characterization: Structure (XRD); Microstructure (SEM); Surface area using BET
Characterization: Thermal (DSC-TG)
28
Reactants used for preparation of nano-sized MgAl2O4 spinel were magnesium
nitrate [Mg(NO)3.6H2O], aluminium nitrate [Al(NO3)3.9H2O], citric acid [C6H8O7.H2O]
and ammonia (NH4OH) solution. All chemicals were used as-received. A stoichiometric
amount of magnesium nitrate, aluminium nitrate and citric acid were dissolved in
distilled water. The metal to citrate ratio was maintained at 1:1. The solutions were mixed
thoroughly and slowly evaporated on a heater. The temperature was maintained at 80 °C.
A highly viscous gel was heated at 80 °C for 2h to get blackish powder. The fine powders
were collected and calcined at different temperatures starting from 600 °C to 800 °C. The
gel and calcined powders were characterized using different instrumental techniques
(described in section 3.3).
3.3 General characterization
3.3.1 AP, BD and CCS
Apparent porosity (AP), bulk density (BD) and cold crushing strength (CCS) were
measured as per the standard of IS: 1528, Part-8 (1974), IS: 1528, Part-12 (1974) and IS:
1528, Part-4 (1974) respectively for both tempered and coked samples. Each value of AP,
BD and CCS was of average of five parallel samples.
3.3.2 HMOR
Hot modulus of rupture (HMOR) was determined by the conventional three-point
bending test conforming to ASTM C133-97 [125], using HMOR testing apparatus
(Netzsch 422, Germany). All the specimens for HMOR testing are dried at 110°C after
wet cutting, without pre-firing in air atmosphere. The heating rate for HMOR testing was
5°C/min and the final firing temperature is 1400°C in air atmosphere with a soaking time
of 30 min. The loading rate for HMOR was 0.15 MPa/sec.
The HMOR value was calculated by the following formula:
HMOR = (3W×L) / (2b×d2) 3.1
where “L” is the span length between the lower supporting points (12 cm for all
the tests in this work); “W” is the maximum load when the specimen is broken (kg); “b”
is the breadth (cm) and “h” is the height of the specimen (cm). Each value of HMOR was
the average of five parallel specimens.
29
3.3.3 MOE
Modulus of elasticity (MOE) test was conducted by using non-destructive
ultrasonic test at room temperature and it was measured indirectly by measuring the sonic
velocity within the brick by passing ultrasonic signal through it. This test is of prime
importance in assessing the spalling resistance of the MgO-C refractory bricks. For this
test, the trial samples were cut into 25×25×150 mm and dried into 110°C for 3 h before
testing. The ultrasonic waves were passed into the sample from one end to another end
and time traveled was calculated.
The MOE value was calculated by the following formula:
MOE = (d × v2 × s) 3.2
where “d” is the bulk density of test sample (g/cc), “v” is the length (l) of the test
specimen / time (t) travels (µ.sec) for the ultrasonic waves were passed into the sample
from one end to another end and “s” is the poison’s ratio (0.90 for magnesia-carbon
sample).
3.3.4 TSI
Thermal spalling index (TSI) is the ratio between the modulus of rupture
(HMOR) and the modulus of elasticity (MOE). The HMOR/MOE ratio was adopted as a
simple index to evaluate the thermal spalling resistance.
3.3.5 Oxidation resistance
For oxidation resistance test, cylindrical samples (height = 50 mm,
diameter = 50 mm) were cut from the tempered bricks and placed in an electrically
heated furnace (heating rate of 5°C/min) under ambient condition at 1200°C for 5 h. The
furnace is then cooled down at the rate of 5°C/min. After cooling, the samples were
horizontally cut into two pieces. After oxidation test, the black surface remaining was
measured at eight different locations and the average value was noted down.
30
3.3.6 Rotary slag corrosion test for micron sized spinel added MgO-C bricks
A dynamic slag corrosion test was conducted using rotary furnace (shown in Fig.
3.2) for micron-sized spinel added MgO-C bricks. Corrosion resistance of the samples
was studied by using a conventional gas-fired rotary slag test furnace charged with steel
making ladle slag. Its chemical composition and basicity are given in Table. 3.7. The
corrosion test was carried out at 1650°C in air for 2 h. The reacted slag was refreshed
every 30 min by charging 300 g of new slag to ensure constant slag composition during
the test. After the slag corrosion test, the furnace was cooled naturally to room
temperature. The sections after slag attack are visually compared and corrosion in
millimeter was calculated by measuring the corroded area of the bricks.
Table 3.7: Chemical composition (%) and basicity of the steel making ladle slag
CaO SiO2 Al2O3 MgO Fe MnO CaO/SiO2
53.36 12.94 24.80 5.48 0.81 0.51 4.12
Fig.3.2: Rotary furnace for conducting slag corrosion test for micron-sized spinel added MgO-C bricks
31
3.3.7 Static crucible slag corrosion test for nano sized spinel added MgO-C bricks
Slag corossion test by static crucible test method was carried out for nano spinel
added MgO-C sample at 1650°C for 2 h with steel making ladle slag. Chemical
composition (%) and basicity of the steel making ladle slag are given in Table 3.7. The
sections after slag attack are visually compared and corrosion in millimeter was
calculated by measuring the corroded area of samples.
3.3.8 Pore size distribution
The test samples (cube shape of 10 × 10 × 10 mm3 was cut from the tempered
bricks) were dried at 110°C for 4 h and cooled in desiccator. The test samples were
placed in pycnometer which was inserted in the part of the mercury porosimetric sample
holder machine with a vacuum of 50 µm Hg. Mercury porosimeter has been used to test
the samples with a maximum pressure of 33000 psi. Surface tension and contact angle of
mercury was 485 dynes/cm2 and 130° respectively. Pore size distribution pattern i.e. open
pore volume available for ‘Hg’ intrusion under pressure with respect to pore diameter has
been characterized.
3.3.9 Thermal
Differential scanning calorimetry (DSC) and thermal gravinometry (TG) of the
gel (which was formed during synthesis of nano spinel using citrate-nitrate method) was
carried out on thermal analyzer (Netzch, Germany) with a heating rate of 10° C / min in
argon atmosphere.
3.3.10 Surface area
Surface area of the nano MgAl2O4 powder was determined using Brunauer-
Emmett-Teller (BET) surface area [Quantachrome, USA]. The measured surface area
was converted to equivalent particle size according to the equation: Size from BET = [6000 /
(density × surface area)]. The density of MgAl2O4 was taken as 3.28 g/cc.
32
3.3.11 Phase analysis
Phase analysis of nano MgAl2O4 powder was carried out by X-ray diffraction
pattern (XRD, PANanalytical, Netherland) using Cu-Kα (λ=1.542 Å). The crystallite size
was determined from the X-ray line broadening using Scherrer relation with correction
factor [126].
3.3.12 Microstructure
Thin slices of slag corrosion tested samples were polished using various grades of
abrasive papers and diamond paste. The microstructures of these samples were done
using optical microscopy (LEICA, optical microscopy with image analyzer) at a
magnification of 250.
The morphology of spinel nano powder was performed using scanning electron
microscopy (SEM, model JSM 6480 LV JEOL, Japan). For the preparation of SEM
sample, the powders were dispersed in isopropyl alcohol using ultra sonication bath (20
kHz, 500 W) for half an hour. One drop of the well-dispersed sample solutions were
deposited on the glass slide. This glass slide was coated with platinum using sputtering
and used for microscopy.
33
Chapter 4
RESULTS AND DISCUSSION
This chapter covers three sections. The first section describes the physical and
chemical properties of micron-sized MgAl2O4 spinel added MgO-C bricks. Three
different types of micron-sized spinel such as AR-78, AR-90 and MR-66 were used in
MgO-C bricks in order to standardize the type and amount of preformed spinel. The
second section describes the characterization of nano MgAl2O4 spinel, synthesized by
citrate-nitrate method. The calcined MgAl2O4 nanopowders were incorporated during
fabrication of MgO-C bricks. The physical and chemical properties of nano-sized spinel
added MgO-C bricks, without spinel as well as standardized spinel added MgO-C bricks
are correlated in the third section.
34
4.1 Physical and chemical properties of micron-sized MgAl2O4 spinel added MgO-C
bricks
4.1.1 AP, BD and CCS (before and after coking)
AP, BD and CCS of MgO-C refractories before and after coking with the addition
of spinel types and amounts are given in Table 4.1, Table 4.2 and Table 4.3, respectively.
Table 4.1: AP (before and after coking) of MgO-C refractories with the addition of
micron-sized spinel
Table 4.2: BD (before and after coking) of MgO-C refractories with the addition of
micron-sized spinel
Table 4.3: CCS (before and after coking) of MgO-C refractories with the addition of
micron-sized spinel
Spinel type / amount
AP in % (before coking) AP in % (after coking) ZS AR-78 AR-90 MR-66 ZS AR-78 AR-90 MR-66
0 2.83 - - - 9.49 - - - 5 - 3.43 6.32 4.66 - 10.65 12.44 10.91 10 - 4.96 4.78 6.04 - 10.12 10.99 10.32 15 - 4.67 8.08 9.87 - 10.94 12.57 11.09 20 - 10.24 8.98 11.11 - 12.06 12.44 11.63 25 - 8.46 8.09 7.7 - 11.79 9.33 11.63
Spinel type / amount
BD in g/cc (before coking) BD in g/cc (after coking) ZS AR-78 AR-90 MR-66 ZS AR-78 AR-90 MR-66
0 2.95 - - - 2.86 - - - 5 - 2.98 2.93 2.98 - 2.9 2.85 2.91 10 - 2.94 2.95 2.95 - 2.87 2.89 2.89 15 - 2.99 2.93 2.89 - 2.9 2.84 2.92 20 - 2.86 2.89 2.83 - 2.85 2.83 2.84 25 - 2.94 2.93 2.92 - 2.87 2.89 2.86
Spinel type / amount
CCS in kg/cm2 (before coking) CCS in kg/cm2 (after coking) ZS AR-78 AR-90 MR-66 ZS AR-78 AR-90 MR-66
0 284 - - - 220 - - - 5 - 310 278 289 - 169 145 134 10 - 418 378 326 - 174 207 165 15 - 363 309 363 - 217 108 202 20 - 249 285 254 - 103 150 128 25 - 262 267 314 - 148 138 153
35
With addition of spinel, the percentage of AP after coking was nearly two to three
times more than that of AP before coking. The percentage of AP after coking was found
to be nearly in the range of 10 % to 12 % (see Table 4.1). BD of MgO-C bricks before
and after coking was in the range between 2.8 g/cc and 2.95 g/cc. However, the BD after
coking was slightly lower as compared to BD before coking (see Table 4.2). As seen
from the Table 4.3, the CCS value after coking was also lower as compared to CCS value
before coking for different spinel added MgO-C bricks. The higher AP and lower BD as
well as CCS after coking was due to the breaking of interlocking texture that has been
created after polymerization of phenolic resin [127]. The breaking of the interlocking
texture was due to the burning out of total organic portion of resin and release of harmful
decomposition gases such as benzene, toluene, phenols and xylenol. [128, 129]. Thus the
matrix phase was loosened thereby reducing strength of the bricks [47]. From AP, BD
and CCS, it was still difficult to choose the appropriate spinel type and amount, so as to
get better properties of MgO-C bricks. Thus, other physical and chemical
characterizations of micron-sized spinel added MgO-C bricks have been done and the
results are discussed in detail.
4.1.2 HMOR and TSI
Figure 4.1 shows HMOR and TSI as a function of different types and amounts of
micron-sized spinel added MgO-C bricks. Higher HMOR was observed for micron-sized
spinel containing bricks, when compared with ZS bricks. Irrespective of spinel addition,
TSI of either 5 wt% or 10 wt% spinel added MgO-C brick was higher as compared to ZS
bricks. The lowest HMOR value for AR-90 (5 wt%) was mainly due to the uncontrolled
volume expansion caused due to in-situ spinel formation [93]. AR-78 (10 wt %) spinel
added MgO-C bricks shows highest spalling index than the other spinel added bricks due
to presence of micro-cracks (formed due to mismatch of thermal expansion co-efficient
between magnesia and spinel) that acts as crack arresters, thus improving the TSI.
However, the HMOR value of different types spinel (10 wt %) added MgO-C bricks was
found to be around 40 kg/cm2. Highest HMOR was obtained for 15 wt % AR-78 spinel
added MgO-C bricks. The reason for high HMOR was due to the formation of controlled
in-situ spinel, thereby reducing pore size and resulting densification [93].
36
3
6
9
12
15
10
20
30
40
50
60
70
80
ZS AR 90 (5%)
AR 90 (10%)
AR 90 (15%)
AR 90 (20%)
AR 90 (25%)
TSI
HM
OR
(kg/
cm2 )
HMORTSI
3
5
7
9
11
13
15
10
20
30
40
50
60
70
80
ZS AR 78 (5%)
AR 78 (10%)
AR78 (15%)
AR 78 (20%)
AR 78 (25%)
TSI
HM
OR
(kg/
cm2 )
HMORTSI
In addition to that, dispersion and retention of carbon after firing in the matrix of
MgO-C refractory was effective in lowering their modulus of elasticity and thus improve
the spalling resistance [130].
37
3
5
7
9
11
13
15
10
20
30
40
50
60
70
80
ZS MR 66 (5%)
MR 66 (10%)
MR 66(15%)
MR 66 (20%)
MR 66 (25%)
TSI
HM
OR
(kg/
cm2 )
HMORTSI
Fig. 4.1: HMOR and TSI as a function of different types and amounts of micron-sized
spinel added MgO-C bricks
4.1.3 Oxidation resistance
Figure 4.2 shows black surface remaining in percentage after oxidation resistance
test for different spinel added MgO-C bricks. It was observed that the brick containing
AR-78 spinel (10 wt % and 15 wt %) was the most effective in prevention of oxidation as
compared to other spinel added MgO-C bricks. The results of oxidation tests in air are
influenced by the permeability of decarburized layer. It was also possible that lower
spinel content samples exhibited greater densification and higher strength after oxidation.
Addition of spinel modifies the pore size distribution (reduces the number of large and
channel pores) thereby hinders the entrance of oxygen into the matrix which ultimately
results in high carbon retention. A similar type of phenomenon was also observed by
Zhang et al [67] and Sen et al. [130].
38
80
85
90
95
5 10 15 20 25
Spinel (wt%)
Bla
ck s
urfa
ce re
mai
ning
(%) ZS
AR-90
AR-78
MR-66
Fig. 4.2: Black surface remaining in % after oxidation resistance test for different bricks
4.1.4. Rotary slag corrosion
Figure 4.3 shows the surface pattern of different spinel added MgO-C bricks after
slag corrosion test. The rotary slag test result was clearly indicated that the incorporation
of AR-78 spinel dramatically inhibits the slag corrosion and penetration resistance
followed by MR-66 and AR-90 spinel addition. The inhibition of slag penetration by
graphite can be simply observed from the residual slag coating on the surfaces of the slag
tested bricks. In this present work, AR-78 spinel added in the form of fine powder (- 45
µm) helps in retarding the slag intrusion and in consequence enhances the corrosion
resistance of the MgO-C refractories. Fine spinel powders generally lead to better slag
penetration resistance than the use of coarse grains as the distribution of fine spinel
powder in the matrix was better and due to their high surface reactivity they can absorb
ions such as Fe2+ and Mn2+ from slag and forming complex spinel more efficiently than
coarse spinel grain [131]. The level of spinel addition also plays an important role in
inhibiting slag penetration and corrosion resistance. If appropriate amounts of spinel are
used, both slag penetration resistance and corrosion resistance of the refractory can be
improved [82, 89, 132].
39
Fig. 4.3: Surface pattern of different spinel type MgO-C bricks after slag corrosion test
4.1.5 Corrosion
Figure 4.4 shows corrosion (mm) as a function of different spinel added MgO-C
refractories. It was clearly indicated that the AR-78 spinel added bricks had undergone
lowest corrosion as it effectively depletes the slag of Fe2+ and Mn2+ cations, thereby
making the slag more viscous, less penetration at the slag brick interface. It was seen
from literature [82, 89, 132] that in-situ spinel formation in the matrix during application
show excellent corrosion resistance in refractories. It was also seen that both slag
penetration resistance and corrosion resistance of the refractory depend on the amounts of
spinel.
Fig. 4.4: Corrosion (mm) as a function of different spinel added MgO-C refractories
5.74
5.745.74
5.15
4.59
6.80
4.65
2.913.
35
0
2
4
6
8
10
AR-90 AR-78 MR-66
Cor
rosi
on (m
m)
0%
5%
10%
5.74
5.745.74
5.15
4.59
6.80
4.65
2.913.
35
0
2
4
6
8
10
AR-90 AR-78 MR-66
Cor
rosi
on (m
m)
0%
5%
10%
40
4.1.6 Pore size distribution
The pore size distribution with average pore diameter of AR-78 and without
spinel added MgO-C brick, after tempering is given in Table 4.4. It was observed that the
average pore diameter of AR-78 was less as compared to without spinel added brick.
Micron sized spinel present in the matrix occupies the pores and voids thereby decreasing
the average pore diameter and improving the texture of the brick. The large pore (>70
µm) distribution was more in without spinel added brick than AR-78 added brick. The
larger pores of 10 µm and above are the path for slag penetration which leads to
penetration of slag into the bricks causing disintegration of MgO grains and finally
dissolution of grains into the slag resulting in removal of the brick layer, which in turn
determines the life of the ladle [133, 134]. The larger amount of fine pores (<1 µm)
provides an important boost in the brick strength as well as effectively restricts the slag
penetration into pores. The emerging microstructure effectively reduces the slag
corrosion of the product in service, as it was difficult for slag to penetrate into pores. At
high temperatures, an organized microstructure with fine pores serves as an efficient heat
insulator, which lowers heat losses of the ladle shell.
Table 4.4: Distribution of pores in MgO-C bricks after slag corrosion
Distribution of Pores (%)
MgO-C Bricks
ZS AR-78 (10 wt %)
< 0.1 µm 5.38 4.73 0.1 µm 20.43 25.82 1 µm 67.38 64.00 10 µm 4.30 3.27
> 70 µm 2.51 2.18
Average pore diameter (µm) 0.3149 0.1309
41
4.1.7 Microstructure
A typical normal and large crystal of 97 % fused magnesia grains are shown in
Fig. 4.5 (a) and (b), respectively. The grain size of large crystal was in the range of 500
µm to 1000 µm. The larger size of the periclase crystals have lower the wear rate and
better the corrosion resistance [58, 59].
Fig. 4.5: Optical micrographs of (a) normal and (b) large crystal of 97 % fused MgO
Figure 4.6 show optical micrographs of without spinel added MgO-C bricks
which indicate (a) crack formation and (b) disintegration of MgO grains after rotary slag
corrosion test. When the slag comes and contact with MgO-C brick, fracture and
disintegration of MgO grains took place due to thermo mechanical stress [100].
42
Fig. 4.6: Optical micrographs of MgO-C bricks without spinel addition after slag corrosion test which indicate (a) Crack formation and (b) disintegration of MgO grains.
The presence of graphite in the matrix after slag corrosion test for AR-78 (10
wt%) spinel added an MgO-C brick was clearly observed from the optical micrograph of
the slag-refractory interface which was shown in Fig. 4.7. The slag has penetrated the
refractory material in pores and cracks. The corrosion of oxides often occurs not only by
dissolution or evaporation of oxide, but also by the penetration of slag into the pores of
the brick. The slag penetrates into the open pores by capillary forces and the solid from
the slag diffuses both through the grain boundaries and into the bulk of the solid [100].
43
Fig.4.7: Optical micrograph shows graphite intact for AR-78 spinel added MgO-C bricks after slag corrosion test
Figure 4.8 (a) and (b) shows the optical micrographs of without spinel and AR-78
spinel added MgO-C bricks after rotary slag corrosion test, respectively.
(a)
44
Fig. 4.8: Optical micrographs of MgO-C bricks (a) without spinel and (b) with AR-78 spinel after slag corrosion test
Dissolution of MgO grains into slag was high in case of without spinel added
MgO-C brick (see Fig. 4.8 a). However, retention of graphite in the matrix and less
dissolution of MgO grains were observed in AR-78 added MgO-C brick (see Fig. 4.8 b).
Slag coating was also observed in case of AR-78 added MgO-C brick thereby hindering
further penetration.
4.1.8 Summary
Out of the three different spinels (AR-90, AR-78 and MR-66) added MgO-C
bricks, AR-78 (10 wt %) spinel added MgO-C brick exhibits better thermal
spalling resistance, corrosion and oxidation resistance as compared to that of AR-
90 or MR-66 spinel added MgO-C bricks.
(b)
45
4.2 Characterization of MgAl2O4 spinel nanopowders synthesized by citrate-nitrate
method
4.2.1. Thermal analysis
Figure 4.9 shows DSC-TG curve of the gel. The weight of the gel decreases as the
temperature increases. The initial ~ 12 % weight loss, which occurs from room
temperature to 150 °C was due to the removal of free water molecules and volatile
residues present in the precursor sample. This weight loss was supported by the presence
of endothermic peak in DSC curves at ~ 122 °C. The second and major weight loss of
around 30 % in the temperature range 150 °C to 900 °C was associated with a broad
exothermic effect. The spinel phase formation was observed by an exothermic peak in the
temperature range from 700 °C to 850 °C as seen in DSC curves. For further
confirmation of the phase transformation behavior, the as-prepared amorphous spinel
powders were heat-treated at different temperatures and phase analysis was done by
XRD.
Fig. 4.9: DSC-TG curve of the gel
46
4.2.2. Structure and microstructure
Figure 4.10 shows the XRD patterns of the as-prepared spinel powder heat-treated
at 600 °C, 700 °C and 800 °C for 5h. It was observed that the spinel powder was in
amorphous nature up to 600 °C. The spinel phase formation starts at 700 °C and pure
MgAl2O4 powder with crystallite size of around 15 nm was found to be formed at 800 °C.
Fig.4.10: XRD patterns of as-prepared spinel nanopowders calcined at different temperatures
47
In order to get a direct and complete picture of the morphology, SEM was
performed for MgAl2O4 nanopowders. Figure 4.11 shows SEM micrograph of calcined
(800 °C) MgAl2O4 nanopowder. The particles size was found to be in the range of 50 nm
to 100 nm.
Fig.4.11: SEM micrograph of MgAl2O4 nanopowder
4.2.3 Surface area
The surface area of the synthesized spinel powder calcined at 800°C for 5 hrs was
found to be nearly 56 m2g-1, which is equivalent to a crystallite size of 15 nm. The
measured surface area was converted to equivalent particle size of 33 nm.
4.2.4 Summary
Nanocrystalline MgAl2O4 spinel powder was successfully synthesized through
citrate-nitrate process. The initial crystallization temperature of the MgAl2O4 spinel
powder was 700°C. Pure MgAl2O4 spinel formation took place at around 800 °C. The
measure surface area was found to be around 56 m2g-1. The crystallite size of MgAl2O4
was found to be around 15 nm. The calcined nanopowders with different concentration
were incorporated in MgO-C refractory and the properties of these refractory are
discussed in next section.
48
4.3 Physical and chemical properties of without, standardized and nano-sized
MgAl2O4 spinel added MgO-C refractory
4.3.1 AP, BD and CCS (before and after coking)
AP, BD and CCS of nano-sized MgAl2O4 added MgO-C refractories before and
after coking are given in Table 4.5. These results were correlated with the results of ZS
and standardized micron-sized 10 % AR-78 spinel added MgO-C refractories.
Table 4.5: AP, BD and CCS of nano-sized MgAl2O4 spinel added MgO-C refractories,
correlated with ZS and AR-78 added MgO-C refractory.
The percentage of AP for ZS and AR-78 added spinel MgO-C bricks was found to
be around 8 % to 9 % after coking which was nearly two to three times more than AP
before coking. With addition of nano spinel in MgO-C refractory, the percentage of AP
varies from 7 to 11 after coking. The higher value of AP may be due to changes in
granulometry and lower value of AP may be due to densification. However, the BD value
was not varied with nano spinel addition and found to be in the range between 2.87 to
2.95 g/cc. CCS after coking was better for the refractory containing 0.5 % and 1 % nano
spinel added refractory. The use of nanoparticles reduces the AP and increases the CCS
of the refractory as compared to ZS and micron-sized AR-78 spinel added bricks. This
may be due to the reason that nano-particles and resin binder disperses among the spaces
between the coarse, medium and fine particles of the refractory matrix. Additives as well
as other miscellaneous materials consequently play a role by filling up the interior pores
and gaps between various particles. [21]. However with increase in nano spinel amount
Spinel amount/ type
Before coking After coking
AP in %
BD in g/cc
CCS in Kg/cm2
AP in %
BD in g/cc
CCS in kg/cm2
ZS 4.03 2.94 398 9.13 2.89 227 10% AR78 3.25 2.92 455 8.23 2.87 266 0.1% NS 4.18 2.95 394 10.25 2.90 235 0.5% NS 2.97 2.96 482 8.01 2.90 278 1.0% NS 2.88 2.95 495 7.87 2.90 288 1.5% NS 4.28 2.94 403 11.89 2.89 240
49
beyond 1 wt% in the matrix part builds up a “bridge” structure results in reduction in
strength and increase in apparent porosity. The similar kind of phenomena was observed
while adding above 1wt% of nano calcium carbonate in corundum based castables [135].
Hence, it was more interesting to characterize the other physical and chemical properties
of these nano spinel added MgO-C refractory.
4.3.2 HMOR and TSI
Figure 4.12 shows HMOR and TSI as a function of nano spinel added MgO-C
refractory. Highest HMOR and TSI were obtained for AR-78 (10 wt %) spinel and 1 wt
% nano spinel added MgO-C refractory. This may be due to the dispersion of nano-sized
spinel in the matrix that protects the carbon [130, 18, 136]. Higher amount of carbon
retention was observed by the addition of spinel in the brick which lead to lowering of
MOE values. Retention of carbon after firing in the matrix of MgO-C refractory was
effective in lowering their modulus of elasticity and was therefore expected to improve
their spalling resistance.
Fig. 4.12: HMOR and TSI as a function of spinel added MgO-C refractory
5
10
15
20
10
20
30
40
50
ZS AR-78 (10 wt%)
NS (0.5 wt%)
NS (1.0 wt%)
TSI
HM
OR
(kg/
cm2 )
HMORTSI
50
4.3.3 Oxidation resistance
Figure 4.13 shows black surface remaining (%) after oxidation resistance test for
nano spinel added MgO-C refractory. It was observed that the brick containing nano
spinel was most effective in prevention of oxidation as compared to without spinel added
or AR-78 spinel added samples due to its high reactivity and high surface area [82; 131].
Better oxidation resistance was obtained for the MgO-C refractory containing 0.5 wt %
and 1 wt % nano spinel.
40
50
60
70
80
90
100
ZS AR 78 (10 wt%)
NS (0.1 wt%)
NS (0.5 wt%)
NS (1.0 wt%)
NS (1.5 wt%)
Bla
ck s
urfa
ce re
mai
ning
(%)
Fig. 4.13: Black surface remaining in % as a function of spinel addition in MgO-C
refractory
4.3.4. Static crucible slag corrosion
Fig.4.14 shows the cross section of the refractory containing without and with
spinel (micron as well as nano) after slag corrosion test. Incorporation of nano spinel (0.5
and 1 %) dramatically inhibits the slag corrosion and penetration resistance followed by
without and AR-78 spinel addition. Spinel being a defective structure, always has a
tendency of substitutional solid solution by accommodating the Fe2+ and Mn2+ and Ca2+
ions from the slag in its defect structure making the slag more viscous and less
51
penetrative. Hence the penetration of slag is reduces which inturn improve the corrosion
resistance. This effect is more predominant when the particle size of spinel decreases to
nano level as the reactivity, surface area and surface volume increase by many folds.
Nano spinel being very fine in nature possesses high reactivity, high surface area and
specific volume thereby forming a coating on the surface of graphite leading to
prevention of decarburisation of graphite from the matrix [82, 131]. Hence, addition of
0.5 wt % and 1 wt% nano spinel in MgO-C refractory exhibits higher oxidation thereby
leading to better slag corrosion and penetration resistance.
Fig. 4.14: Surface pattern of different spinel type MgO-C samples after slag corrosion test
4.3.5 Corrosion
Figure 4.15 shows corrosion (mm) as a function of spinel added MgO-C
refractories. It was clearly indicated that the 1 wt % nano spinel added refractory had
undergone lowest corrosion than the other type of spinel added bricks. Fine spinel
ZS
NS (1.0 wt%)
NS (0.1 wt%)
NS (0.5 wt%)
AR-78 (10 wt%)
52
powders generally lead to better slag penetration resistance than the use of coarse grain
due to its high surface reactivity [82]. Nano spinel powders can take up slag ions such as
Fe2+ or Mn2+ and dissolve more efficiently than the coarse spinel grain [131]. Zhang et
al., [67] reported that the spinel in the matrix of MgO-C refractory could effectively
protect the graphite against oxidation by bonding the graphite flakes, and subsequently
increase the corrosion resistance. This also maintained the integrity of the refractory
texture and thus inhibits further slag penetration and subsequent corrosion. Hence
refractories with spinel addition demonstrate a better corrosion resistance along with
matrix densification as compared to bricks without spinel addition. Addition of nano
spinel further leads to densification of texture owing to its finer size and surface
reactivity. Several papers have reported that the type, size and amount of spinel addition
play an important role in inhibiting slag penetration and corrosion resistance [82, 89,
132].
0
2
4
6
8
ZS AR 78 (10 wt%)
NS (0.1 wt%)
NS (0.5 wt%)
NS (1.0 wt%)
NS (1.5 wt%)
Co
rro
sio
n (m
m)
Fig. 4.15: Corrosion (mm) as a function of spinel added MgO-C refractories
53
4.3.6 Pore size distribution
Table 4.6 shows the distribution of pores and average pore diameter of without
spinel, AR-78 and nano spinel added refractory after tempering. Pore size and number of
pore was decreased by maintaining a uniform size distribution by incorporating a small
amount of nano-sized spinel in the matrix of refractory. The nano-sized spinel goes into
the pores and in between the macron-sized magnesia and graphite particles. As a result,
the pore size and number of pores are reduced and hence a uniform pore size distribution
was created in the matrix [130]. The average pore diameter for 1 wt % nano spinel added
MgO-C brick was found to be less as compared to without and 10 wt % AR-78 added
spinel bricks.
Table 4.6: Distribution of pores in spinel added MgO-C refractories after slag corrosion
Properties 0% ZS
10% AR-78
0.5% NS
1% NS
Avg. pore dia. (µm) 0.2763 0.1309 0.1237 0.0804
Pore size distribution
< 0.1 µm (%) 9.48 19.17 16.03 22.37
0.1 – 1 µm (%) 38.24 48.33 48.09 46.49
1-75 µm (%) 48.69 30 33.21 28.95
>75 µm (%) 3.27 2.08 2.29 1.75
4.3.7 Microstructure
Figure 4.16 (a) and (b) shows the optical micrographs of 0.5 % and 1 % nano
spinel added MgO-C refractory, respectively after rotary slag corrosion test. Dissolution
of MgO grains into slag was observed in both without spinel and AR-78 spinel added
MgO-C brick [see Fig. 4.8 (a) and (b)]. However, with addition of 1 wt % nano spinel
restricted dissolution of MgO grains as well as retention of carbon in the matrix (see Fig.
4.16 b), thus gives better corrosion resistance.
54
Fig. 4.16: Optical micrographs of (a) 0.5 % and (b) 1 % nano spinel added MgO-C refractories after slag corrosion test.
4.3.8 Summary
The average pore diameter was less for 1 wt % nano spinel added MgO-C refractory
as compared to ZS and AR-78 added spinel bricks. Large pores (1-75 µm and above)
are less in case of 1 wt % nano spinel added brick as compared to others.
Low corrosion rate was observed in case of 0.5 wt % and 1 wt% nano spinel added
refractory.
0.5 wt % and 1 wt % nano spinel addition in MgO-C refractory gives better HMOR,
TSI and corrosion as well as oxidation resistance as compared to that of ZS and AR-
78 added bricks.
(a)
(b)
55
Chapter 5
CONCLUSIONS
56
The present work deals with the improvement of physical and chemical properties
of MgO-C bricks with the addition of micron and nano-sized MgAl2O4 spinel. The
significant findings of this work are
Oxidation, slag penetration and thermal spalling resistance of near stoichiometric
spinel (10 wt% AR-78) added MgO-C refractory brick properties are found to be
superior than AR-90, MR-66 and without spinel added bricks, which generally
determine the life of the ladle. This was due to the presence of large amount of fine
pores and modification of pore size distribution by spinel addition and effectively
depleting the divalent cations (Fe2+, Mn2+) from the slag coming in contact with the
brick surface thereby making the slag more viscous and less penetrative which in
turn helped in prevention of slag penetration.
Carbon retention in AR-78 (10 wt %) spinel added brick was high (observed from
oxidation test) as compared to AR-90, MR-66 and without spinel added MgO-C
bricks. Thus, spinel addition helped in preventing decarburization of graphite present
in the brick.
Slag coating on AR-78 added MgO-C bricks (observed from rotary slag corrosion
test) led to further prevention of penetration of slag into brick.
AR-78 (10 wt%) spinel added bricks possesses greater degree of thermal spalling
resistance as compared to other spinel or without spinel added bricks. This was due
to mismatched thermal expansion coefficient between spinel and magnesia which led
to the formation of voids and micro cracks in the refractory body. The voids and
micro cracks present in the matrix acts as crack arresters, thereby improving the
thermal spalling resistance of bricks.
The nano-sized MgAl2O4 spinel was synthesized using citrate-nitrate route and the
crystallite size and particle size was found to be around 15 nm (calculated from
XRD) and 32-50 nm (observed from SEM) respectively at 800 °C. These powders
were used in fabrication of nano spinel added MgO-C bricks. The average pore
diameter was less in nano spinel added MgO-C bricks as compared to micron spinel
57
bricks. The carbon retention was higher in nano spinel added MgO-C bricks. Thus
nano spinel (0.5 wt % and 1 wt %) added bricks gives better HMOR, TSI, oxidation
and slag corrosion resistance as compared to AR-78 or without spinel added bricks.
Graphite retention and corrosion resistance has been improved by many folds with
the addition of nano spinel as compared to without spinel and 10 wt% AR-78 spinel
added bricks which was clearly observed from optical micrographs.
Finally, this research work clearly shows the potential of micron and nano MgAl2O4
spinel added MgO-C bricks for the application in the slag lines of ladle metallurgical
furnace.
SCOPE FOR FUTURE WORK
The results so far obtained are highly encouraging and the main suggestions for
scope for future work are as follows:
Exploration of different sources to get bulk amount of nano spinel.
Field trials to be carried out with the developed AR-78 spinel added MgO-C bricks
in steel ladle.
Use of cost effective prespin (preformed spinel) in MgO-C refractories and study its
effect on the physical and chemical properties.
Mixing of nano spinel along with other ingredients of MgO-C brick in the mixture
machine was very difficult and cumbersome. So innovative technology was to be
developed to ease the incorporation of nano spinel in MgO-C refractory system.
58
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Curriculum Vitae
Name : Mrs. Rashmi Rekha Das Date of Birth : 25th December 1978 Sex : Female Marital Status : Married Nationality : Indian Address for Communication : Mrs. Rashmi Rekha Das
Qr. No: P-27, TRL Township Belpahar (Post), Jharsuguda (District) Orissa – 768 218
Education Qualification :
Examination Board Year of Passing Marks (%) Division
AICeram (Equivalent to B.Tech -Ceramics)
Indian Ceramic Society
2006 63.79 First
Diploma (Ceramic Tech.,)
SCTE & VT 1999 74.21 First (Hons.)
SSLC CBSE 1994 76.40 First
Industrial Experience :
(i) Working as Officer (R&D) in M/s. Tata Refractories Ltd., Orissa, India since Dec’2004 to till date (Domain: Testing and instrumentation, New product development).
(ii) Served in M/s. Tata Refractories Ltd., Orissa, India as Officer (R&D) from August 2001 to December.2003 (Domain: In-process quality control). .
(iii) Apprenticeship Trainee at M/s. NALCO, Orissa, India from January 2001 to August 2001.
(iv) Technical training on application of refractories in steel plant at M/s.TISCO through SNTI.
Languages Known : Fluent in reading, writing and speaking English, Hindi, Oriya
Extra Curricular Activities : Sketching, Singing, Playing badminton and community development work Computer Knowledge : Well versed with personal computer
Publications resulting from the M. Tech (R) work
1. Rashmi R Das, Bibhuti B. Nayak, S. Adak, A. K. Chattopadhyay, "Effect of spinel addition in MgO-C refractory for slag zone of steel ladel", Technical proceedings in IREFCON 10, pp-155-159 (2010).
2. Rashmi R Das, S. Adak, A. K. Chattopadhyay, Bibhuti B. Nayak, “Influence of nanocrystalline MgAl2O4 spinel addition on the properties of MgO-C refractories” (Communicated in Materials and Manufacturing Processes).
(Rashmi Rekha Das)
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